AFOs are commonly prescribed as a form of orthotic intervention to improve gait of patients with neuromuscular diseases, such as stroke, cerebral palsy or brain injury. They are primarily designed to provide adequate plantarflexion resistance, to prevent foot-drop and to provide sufficient medio-lateral stability for a pathologic ankle joint. Therefore, quantitative information on stiffness of an ankle joint and an AFO in the sagittal and coronal planes is essential in an AFO prescription.
An understanding of stiffness properties inherent in the design of an AFO is important because it is the key in preventing an under- or over-prescription and in providing an optimum AFO for a patient. However, the complicated geometry of a thermoplastic AFO and its interaction with the lower limb make it difficult to evaluate the mechanical behavior of an AFO during gait. This has been one of the limitations for the development of a prescription system to provide the most appropriate AFO to patients with various medical conditions.
Spasticity is one of the most common neurological impairments and occurs after the lesion of the upper motor neuron (UMN) in patients with hemiplegia. It is defined as disordered sensori-motor control resulting from a UMN lesion and presenting as intermittent or sustained involuntary activation of muscles. Some studies indicate that about 36% to 38% of patients with stroke developed spasticity during the first year. Spasticity generally induces an increase in ankle joint stiffness and foot-drop during ambulation. Therefore, patients with a spastic ankle joint are prevalently prescribed with an AFO.
Joint stiffness is attributed to a reflex stiffness component which is caused by the alternation in the muscular activation level and a non-reflex stiffness component which stems from the mechanical properties of the muscles, joints and tendons. Both reflex and non-reflex changes occur at an ankle joint in patients with hemiplegia.
A clinician usually evaluates ankle joint stiffness of a patient manually and attempts to reflect its input for decision making in the design of an AFO. However, this protocol heavily relies on the anecdotal or individual clinical experience. In the laboratory setting, a number of sophisticated experimental devices have been introduced to assess resistive torque and stiffness of a spastic ankle joint, such as a device disclosed in U.S. Pat. No. 6,599,255 B2. However, these devices might not be very practical in the clinical setting due to their size, costs and complexity.
In a clinical environment, a manual device is more practical. They can minimize discomfort and injuries and are considered safer than automated devices and easily applied in a clinical environment. However, they have the disadvantage that the applied force, velocity, and acceleration achieved as well as the point of force application and their orientation are difficult to control.
Traditionally, it is believed that stiffness of an ankle joint and an AFO should be considered in an AFO prescription. There is no available device to perform this function. Accordingly, there is a need for a manual device which can measure stiffness of both an ankle joint and an AFO quantitatively in order to assist a clinician in an AFO prescription. It would be beneficial for a patient if a clinician is able to prescribe an AFO based on the objective information on stiffness of an ankle joint and an AFO instead of a subjective decision that may be based on their experience.